The chemical deposition of thin solid films from gaseous (vapor-phase) chemical precursors onto solid substrates is of great interest in many areas including semiconductor fabrication, magnetic data storage, nanotechnology and others. In particular, atomic layer deposition (ALD) and chemical vapor deposition (CVD) processes are commonly used to deposit both dielectric and metal films onto semiconductor substrates. Increasingly, these applications require that the deposited film meet strict standards for thickness uniformity across the substrate and repeatability in such thicknesses over multiple substrates, while at the same time the process equipment is required to provide high film deposition rates so as not to present a bottleneck in the overall fabrication process.
In order for CVD and ALD equipment to meet such requirements, the flux of vapor precursors to the substrate must be tightly controlled and shaped. Often, there can be multiple gaseous precursors that must react to form the desired film and all must be delivered to the substrate in a precise and controllable manner. In some cases, it is advantageous to mix these multiple precursors together prior to introducing them into the reactor chamber. In other cases, it is preferable to maintain the precursors isolated from one another until they come into contact with the substrate so as to prevent any unwanted premature reactions.
Generally, uniform precursor flow into the reaction chamber is attempted by providing a flat plate with many small holes in between the gas-source and the substrate (a so-called showerhead). An early description of a device for providing such axial-symmetric gas flow towards a substrate is provided in U.S. Pat. No. 4,798,165 of deBoer et al. The diffusion plate or showerhead can have separate zones such that some holes are used for introducing one precursor and other holes are used for introducing the other precursor. In this way the precursors are kept separate so that no mixing occurs prior to the precursors entering the reaction space adjacent to the substrate.
One such showerhead is described in U.S. Published Patent Application 2006-0021703 of Salvador P. Umotoy. In this design, the showerhead faceplate has a number of gas passageways to provide a plurality of gases to the process region without commingling of those gases. A gas distribution manifold assembly is coupled so as to provide the different gasses to the various gas holes in the faceplate.
Another design for maintaining gases in separate passageways until they exit the distribution plate into the process region is described in U.S. Pat. No. 5,595,606. This showerhead includes a multiple block stack that ostensibly maintains two gases in separate passageways until they exit the distribution plate into the process region.
While showerheads of the sort described above purport to maintain separation of the various gases used in the ALD and CVD the present inventors have observed that if the relative flow rates of the different precursors flowing through adjacent holes are not well designed, recirculation can occur along the showerhead faceplate between the holes. FIG. 1 illustrates this condition. Shown in the diagram is a cut away view of a showerhead apparatus 10 having two individual gas manifolds generally indicated at 12 and 14. The upper manifold 12 includes gas passageways 16a and 16b, which provide means for the gas in manifold 12 to exit via holes 18a and 18b in the faceplate 20 of showerhead 10. Similarly, the lower manifold 14 includes gas passageways 22a and 22b, which provide means for the gas in manifold 14 to exit via holes 24a and 24b in faceplate 20.
As shown, recirculation of the different precursor gases has been known to occur along the showerhead faceplate 20 between the holes associated with the different manifolds 12 and 14. This undesired mixing of the precursors can cause unwanted reactions therebetween and reduce film uniformity on substrates in proximity thereto. Furthermore, when multiple zones are present within a single showerhead the spacing between the outlet holes of different zones becomes constrained by the number and size of holes required for flow uniformity.
Another problem with such showerhead designs is that it is difficult or impossible to maintain a difference in temperature between the two precursors because they both flow through the same solid plate 26 before reaching the faceplate 20. In many cases, it would be desirable to maintain precursors at different temperatures until they react at the substrate surface.
What is needed, therefore, is a gas distribution system that overcomes these limitations of conventional showerheads.